The buckets of gas turbines are typically cooled by air cooling systems. Compressor discharge air is brought into the wheel and channelled into the dovetail slots between the wheel and turbine buckets through drilled holes. The buckets themselves have one or more air passages for flowing the cooling air along the bucket. While the matching slot arrangement of the wheel and buckets has been effective for the purposes for which they were designed, they do have the disadvantage of affording leakage paths which are detrimental to the performance of the gas turbine. Many modifications to various designs for transferring cooling air from the rotor wheel to the buckets have been proposed in the past. However, the disadvantages have mainly been accepted and the designs have taken into consideration the relatively low pressure and leakages of those cooling air flow paths.
In gas turbines, particularly those operating at very high temperatures, it has now become desirable to employ steam as the cooling medium. However, steam pressures are multiples of those used in air cooling systems. Moreover, leakage paths must be avoided, or at least controlled to an absolute minimum level. In short, the transfer of the steam between the rotor wheel and the buckets must occur with a minimum loss of fluid to the surrounding environment, while maintaining a high degree of reliability to deliver required pressures and flow to the bucket. Additionally, steam used to cool the buckets must be returned to the source, with minimum loss to the closed-cycle cooling system. Thus, two different transfer points per bucket at different pressures and temperatures are required.